Microlithographic processes are typically used to fabricate integrated circuits on semiconductor wafers. A photosensitive layer known as photoresist is applied to the wafer, patterned by exposure to radiation through a pattern transfer tool in an aligner system or aligner, and subsequently developed. The pattern transfer tool typically includes a pattern composed of opaque and transparent areas that are to be transferred to the wafer surface. The tool is inserted into the aligner and is readily removable for interchanging with other tools having other patterns. Radiation energy passes through the pattern transfer tool to the photoresist layer and induces a chemical reaction therein. The exposure continues until a quantity of radiation energy sufficient to induce the chemical reaction throughout the depth of the photoresist layer has been transferred.
Microlithographic processes for fabricating integrated circuits are continually being improved to allow the manufacture of ever smaller devices to increase device speed and density on a chip. Faster and denser chips lead to more powerful and less expensive electronic products. The minimum device size that can be printed by a process is determined by its resolution capability, i.e., the minimum line width the process can print.
The resolution capability of microlithographic processes is ultimately limited by undesirable diffraction effects of the radiation as it passes through the pattern transfer tool. The theoretical smallest distance an aligner can resolve is known as the Rayleigh limit and is described by the Rayleigh equation: ##EQU1## where R.sub.min is the minimum resolution, i.e., the minimum separation of two barely resolvable objects, k1 is a process dependent constant, .lambda. is the wavelength of exposing radiation and NA is the numerical aperture, a measure of the ability of the optics of the aligner to collect diffracted light from the tool and project it onto the wafer. Because the Rayleigh limit is proportional to the wavelength of exposing radiation, one method of improving resolution is to use radiation of a shorter wavelength.
Current aligner systems typically use light of the 436 nm "g-line" or the 365 nm "i-line" of a mercury vapor lamp. X-ray systems, having wavelengths of 0.4 nm to 5.0 nm, are not yet fully developed as production tools. Deep ultra-violet systems, having wavelengths of about 250 nm are now available, but such systems are expensive and not fully proven in a production environment.
Another method for improving resolution is to decrease the value of k1 in the Rayleigh equation by using destructive interference to modify the diffraction pattern. This is accomplished typically by shifting the phase of a portion of the radiation passing through the pattern transfer tool in such a way that the diffraction patterns of the phase-shifted and nonphase-shifted portions destructively interfere. This phase-shift technique can decrease the value of k1 in the Rayleigh equation from approximately 0.60 to approximately 0.35 for some types of "i-line" aligners. Another advantage of the phase-shift technique is that it does not require an expensive new aligner system to improve resolution; the pattern transfer tool alone is modified.
However, radiation of different wavelengths is phase-shifted by different amounts as it passes through the pattern transfer tool. Because the tool is typically tuned to the center wavelength of the illuminating bandwidth, radiation having a wavelength different from the center wavelength will not be phase-shifted by the desired amount. The deviation from the desired amount of phase-shift for a particular wavelength is proportional to the difference between that wavelength and the center wavelength. Therefore, the ability of the radiation to destructively interfere depends upon the light being essentially of a single wavelength, i.e., having a narrow bandwidth.
Narrowing the bandwidth increases, however, the time required to expose the photosensitive layer because the radiation energy is being transferred to the photoresist layer at a slower rate. Because aligners process wafers individually, lengthening the exposure step can create a bottleneck in the entire wafer fabrication process.
Two primary types of aligner systems are used in the fabrication of very large scale integrated circuits--scanning projection aligners and step-and-repeat aligners, or "steppers." Some scanning projection aligners, such as the Micralign.TM. aligner manufactured by SVG Lithography ("SVG-L"), use a pattern transfer tool called a mask that includes a pattern that is transferred to the entire wafer in one printing step. Other scanning systems, such as the SVG-L Micrascan.TM. aligner, use a pattern transfer tool with an enlarged pattern and scan this image several times over a semiconductor wafer in order to transfer the pattern over the entire wafer substrate.
Steppers use a pattern transfer tool called a reticle that typically includes an enlarged pattern that is reduced and transferred to a portion of the wafer. Steppers step across the wafer, repeatedly transferring the image of the reticle at different consecutive locations on the wafer. Another form of stepper, such as an Ultratech.TM. Stepper, uses a pattern transfer tool having the same size image as is printed on the semiconductor workpiece, and steps this image several times upon the workpiece in order to fully cover it.
Bandpass filters are currently used in exposure tools with refractive optics in order to narrow the bandwidth of the exposing radiation. Refractive optics must use a narrow bandwidth of light in order to properly focus the exposing radiation within the optical train. Broadband exposure wavelengths will not focus properly. For this reason, refractive optics will require more time to expose the wafer, for a given light-source power input, than would a broadband, reflective optics systems.
Scanning projection aligners, or those utilizing reflective optics, are designed to pass a relatively broad band of light to achieve light intensities that will more quickly expose the photoresist. These aligners use internal broadband bandpass filters, but such reflective optical systems do not use narrow bandpass filters because mirrors are used to focus an image of the mask onto the wafer, and excessive narrowing of the exposure bandwidth would unnecessarily slow wafer production.